The BC Ministry of Forests, Lands and Natural Resource Operations and Alloycorp Mining (formerly Avanti Mining) have engaged Associated Engineering over the last ten year to inspect and manage the 137 metre five-span Nass River Bridge. Following the identification of deterioration in one of the girders on the 63 metre main span during a close proximity inspection, we recommended the Ministry post the bridge for reduced loads and developed a number of strengthening schemes. As the bridge provides the only access to Avanti’s Molybdenum Mine at Kitsault, Avanti preferred a replacement option and retained Surespan Construction to deliver the project.
Given the length, weight and remote location of the bridge, a typical erection scheme using a large crane to remove the existing girders and place the new bridge superstructure was not possible. Working closely with Surespan Construction, we developed a replacement design utilizing the existing concrete piers and abutments. The bridge incorporated typical industrial bridge details and a modified precast concrete panel design to allow the development of composite action in the negative moment regions.
Together, we developed an innovative bridge demolition scheme where the new steel girders were erected on the existing bridge and then supported on large head frames at each pier location. The existing glulam girders were then hung from the steel girders using steel hangers fitted with sliders, which allowed Surespan Construction to incrementally remove the glulam girders by sliding them out over the steel girders. Once the glulam girders were removed, the new steel towers were lowered onto the existing concrete piers and the girders lowered onto the towers from the head frame.
The project was successfully completed over a three-month period during the winter shutdown thereby minimizing impact on road users and Avanti Mining’s operations.